1. Field of the Invention
This invention relates generally to Josephson tunneling storage devices for use in DRO (Destructive Read Out) random access memory arrays. More specifically, it relates to Josephson tunneling devices which are capable of storing binary information in the form of a single flux quantum. Still more specifically, it relates to single flux quantum Josephson tunneling storage devices which are capable of storing a single flux quantum in the absence of applied bias or at zero applied field. Still more specifically, it relates to a single flux quantum Josephson tunneling storage device in which writing and sensing is performed with coincident currents. Each device during sensing acts as its own sense detector and is capable of switching to the voltage state if a flux quantum is stored. The ability to store a single flux quantum at zero bias is determined by controlling the Josephson current density profile across the junction which, in turn, permits the storing of binary information without the need for external loops or circuits which normally carry circulating currents. Sensing uses the fact that the gain characteristics of the junction have their switching thresholds extended by increasing the junction resistance across the entire Josephson junction. The use of such single flux quantum storage devices permits the fabrication of very high density arrays having extremely fast switching times which require no ultrahigh sensitivity circuits to sense a stored signal.
2. Description of the Prior Art
The principle of Josephson current devices is understood in the prior art, and such devices have been proposed for memory applications. In particular, reference is made to U.S. Pat. No. 3,626,391 issued Dec. 7, 1971, in the name of W. Anacker and assigned to the same assignee as the present invention. In that patent, a memory array is described which includes a plurality of Josephson tunneling devices wherein each memory cell is comprised of two such Josephson devices. The state of each memory cell is determined by the direction of the circulating current in the cell.
In two technical papers, a superconducting ring containing a barrier such as a Josephson junction is studied. In particular, the reaction of superconducting rings having weak links therein to the application of external magnetic fields has been reported by F. Block in a paper entitled "Simple Interpretation of the Josephson Effect", which appears in Physical Review Letters, Vol. 21, No. 17, Oct. 21, 1968 on page 241. The paper discusses the Josephson effect in terms of a superconducting ring wherein the ring is linked with an external magnetic field. In another article by D. E. McCumber appearing in Journal of Applied Physics, Vol. 39, No. 6, May 1968, page 2503, superconductor weak link junctions and the effect of magnetic fields on these junctions is discussed. On page 2507 of this article, McCumber describes a superconducting loop containing a single weak link and mentions that this configuration has potential utility as a memory element.
U.S. Pat. No. 3,705,393 issued Dec. 5, 1972, in the names of W. Anacker and H. H. Zappe and assigned to the same assignee as the present invention, describes a superconducting memory array wherein the memory cells are superconducting rings each of which has at least one element therein capable of supporting Josephson tunneling current. In the patent, coincident currents are used to trap flux in the rings and to release the trapped flux for readout of the memory cells. Fast operation and tolerable limits on drive currents are indicated as being obtainable if single flux quantum operation is utilized. To achieve single flux quantum operation, the capacitance, inductance, and damping of each memory cell must be within certain limits.
In 1970, P. W. Anderson, in an article in Physics Today, Vol. 23, page 29 (1970), described a flux shuttle which is a single vortex shift register. The first experimental results on the flux shuttle were recently reported by T. A. Fulton and L. N. Dunkelberger in an article in Applied Physics Letters, Vol. 22, page 232 (1973). In the devices of the articles and the last mentioned patent, flux is stored either in small superconductive inductances in the form of loops containing one Josephson junction or in single rectangular junctions which require an external bias field. Reading is performed by sensing the signal induced into the array lines during the release of the trapped fluxoid. Although very high packing densities are possible with such schemes, their disadvantage is that the energy released for reading is of the order of only 10.sup.-.sup.18 joules. Also, since the fabrication of the prior art arrangements are relatively large due to the requirement for loops, such arrangements do not provide the ultimate in small size, high speed devices which is the direction towards which most present-day technologies are tending. In addition, none of the known arrangements provide a device which is its own sense detector switching the device to the gap voltage if a flux quantum is stored.